The overall objective of this project is to analyze the immune response of the mouse intestine to the protozoan parasite Giardia muris. Mice infected with this organism provide an animal model of human giardiasis. Two of the specific aims are to establish whether clearance of Giardia muris infection is associated with an increased number of monunuclear leucocytes in the intestinal lumen, and/or with altered proportions of different mononuclear leucocyte populations in this site. The other major aims are to identify the mononuclear leucocyte population which increases in numbers within the intestinal epithelium during giardia muris infection, and to investigate the roles of macrophages and IgA in clearing Giardia muris trophozoites from the intestine. Mononuclear leucocytes will be identified by using a panel of monoclonal antibodies against mouse leucocytes surface antigens (Thy-1.2, Lyt-1, Lyt-2, Mac-1, and, if possible, NK-1.2). The antibodies will be used to identify cells by immunofluorescent staining, and by staining procedures involving the use of visible markers (peroxidase reaction product, and ferritin). Mononuclear leucocyte populations in the intestinal lumen and epithelium will be identified and quantified in uninfected mice, and at serial times after initial infection of mice with Giardia muris cysts. Mixed populations of Giardia trophozoites and mononuclear leucocytes from the intestinal lumen will also be examined, by light microscopy and electron microscopy, for evidence of trophozoite phagocytosis by macrophages, and for ultrastructural evidence of lymphocyte attachment to trophozoites. Trophozoites from the intestinal lumen will be stained for mouse IgA, using an appropriate monoclonal antibody. If IgA is demonstrated on trophozoites, the time-course of its appearance on their surfaces during the infection will be studied. In addition, the question of whether IgA opsonizes trophozoites for macrophage phagocytosis will be investigated. The study will be carried out using immuologically-competent mice, and mice with T lymphocyte deficiency (nude mice) or natural killer (NK) cell deficiency (beige mice). Monoclonal antibodies will also be used to deplete particular populations of mouse mononuclear leucocytes in vivo, and the effect of this depletion on clearance of Giardia muris infection will be evaluated.